US jets will defend favoured rebels in Syria against attack by Assad regime jets, in an extension of their mission which entrenches America and its allies further in the civil war.

The first 'authorised rebel' fighters trained by the US army in Turkey crossed over into Syria last month - although a number of men from the brigade they joined were subsequently kidnapped by al-Qa'ida's local affiliate, Jabhat al-Nusra. One reason the fighters are distrusted by the other rebels is that the United States, as well as bombing Islamic State of Iraq and the Levant (Isil) from the air, is also attacking elements within Jabhat al-Nusra, which has proved the most formidable opponent of the regime and is allied with rebel groups who do not share its ideology.

The other is that the rebels are being specifically trained to fight Isil - not the Assad regime.

The shift in strategy to allow bombing raids to support defensive - though not offensive - actions by rebels against the regime was reported by the 'Wall Street Journal'.

"For offensive operations, it's Isis (Isil) only," it quoted an official as saying. "But if attacked, we'll defend them against anyone who's attacking them. We're not looking to engage the regime, but we've made a commitment to help defend these people."

Since aborting threatened air raids on the Assad regime after the chemical weapons attack on east Damascus in 2013, President Barack Obama has shied clear of targeting government forces. That has gone so far as vetting potential rebel trainees to ensure they regard their main enemy as Isil, not Bashar al-Assad, the Syrian president - something that is true for only a minority of rebel forces.